1. Field of the Invention
The present invention relates generally to a tantalum solid electrolytic capacitor and, in particular but not exclusively, to a method of and an apparatus for manufacturing tantalum solid electrolytic chip capacitors for use in various electronic appliances.
2. Description of the Related Art
Recently, with a tendency to reduce the weight and size, with a tendency to enhance the performance, and with the development of a packaging technique, the number of electronic appliances that are former into chips is rapidly increasing. Also, in the field of solid electrolytic capacitors, with a tendency to reduce the size and increase the capacity, the number of the solid electrolytic capacitors that are formed into chips is increasing. However, a further reduction in size and a further increase in capacity are demanded.
FIG. 9 depicts a conventional tantalum solid electrolytic chip capacitor, while FIGS. 10A and 10B depict an essential portion of a tantalum solid electrolytic capacitor made by a conventional manufacturing method after assemblage.
In these figures, reference numeral 20 denotes a capacitor element, which is a sintered porous body formed of powder of tantalum metal, one of valve action metals. An anode lead 21 made of a tantalum wire is partially embedded in and extends outwardly from the porous body. A dielectric oxide layer 22 is formed on a portion of the anode lead 21 and over the entire surface of the porous body by anodizing. An electrolyte layer 23 such as manganese dioxide or the like is formed on the surface of the dielectric oxide layer 22, and a cathode layer 24 is further formed on the electrolyte layer 23. The cathode layer 24 includes a carbon layer and a silver paste layer laminated one upon another by dipping. Reference numeral 25 denotes a Teflon-made insulating plate mounted on the anode lead 21 to prevent an electrolyte from reaching and adhering to the anode lead 21 during formation of the electrolyte layer 23. Reference numeral 26 denotes an anode terminal, which is connected to the anode lead 21 by welding and is bent after formation of a sheathing resin 28 (explained later). Reference numeral 27 denotes a cathode terminal electrically connected to the cathode layer 24 of the capacitor element 20 via a thermosetting conductive adhesive 29 made of epoxy resin, which is cured by a hot-air circulating dryer. The cathode terminal 27 is bent after the molding of the sheathing resin 28. The sheathing resin 25 is covered over the entire capacitor element 20 by molding.
In the conventional tantalum solid electrolytic capacitor of the above-described construction, the conductive adhesive 29 used to connect the cathode terminal 27 to the cathode layer 24 of the capacitor element 20 is cured by the hot-air circulating dryer during batch processing wherein the conductive adhesive 29 is allowed to stand for 90 minutes in an atmosphere of 180xc2x15xc2x0 C. Accordingly, such much time as about 6 hours is required from the start of assembling to the end of curing of the adhesive, including the time required for increasing the temperature of a product from when the product is introduced into the hot-air circulating dryer to when it reaches 180xc2x15xc2x0 C., the curing time of 90 minutes, and the annealing or slow cooling time up to when the product is taken out to the room temperature. According to this manufacturing method, the conductive adhesive 29 is first discharged on the cathode terminal 27, and the capacitor element 20 is then placed on the conductive adhesive 29. Thereafter, the capacitor element 20 is temporarily fixed by connecting the anode lead 21 to the anode terminal 26, and the curing is carried out using the hot-air circulating dryer. Thus, the capacitor element 20 is merely placed on the conductive adhesive 29, which in turn comes to have a thickness of about 100 xcexcm to about 500 xcexcm, resulting in an increase in apparent resistance of the conductive adhesive itself. Furthermore, as shown in FIG. 10B, it is likely that air is drawn into an interface between the capacitor element 20 and the conductive adhesive 29 to create air bubble portions 30. The presence of the air bubble portions 30 increases the connection resistance, deteriorating the electric characterietics (tan xcex4/1 KHz or ESR/100 KHz) required for a capacitor.
Furthermore, as shown in FIG. 9, because the conductive adhesive 29 is merely coated on the cathode terminal 27, the bonding area between the cathode terminal 27 and the capacitor element 20 is-small and, hence, the bonding strength is low, resulting in insufficient reliability.
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an objective of the present invention to provide a method of and an apparatus for stably manufacturing tantalum solid electrolytic capacitors that are superior in electric characteristics (tan xcex4/1 KHz or ESR/100 KHz).
Another objective of the present invention is to provide the method and apparatus of the above-described type that are superior in productivity.
In accomplishing the above and other objective, the method according to the present invention comprises: preparing a length of metallic lead frame having a plurality of anode terminals and a plurality of cathode terminals; coating the plurality of cathode terminals with a thermosetting conductive adhesive; placing cathode layers of a plurality of capacitor elements on the conductive adhesive; placing anode leads 25 extending outwardly from the plurality of capacitor elements on the plurality of anode terminals respectively; joining the anode leads to the plurality of anode terminals respectively, by welding; applying a pressure to the plurality of capacitor elements so that a portion of the conductive adhesive is squeezed out of one surface of each of the plurality of capacitor elements to a neighboring side surface thereof joining the plurality of cathode terminals to the plurality of capacitor elements, respectively, by heat-curing the conductive adhesive; and covering the plurality of capacitor elements with a sheathing resin.
According to the above-described method, the pressure applied to the capacitor elements moves a portion of the conductive adhesive to the neighboring side surface of each capacitor element, thereby causing the conductive adhesive layer to have a reduced thickness. As a result, the resistance of the conductive adhesive in the thickness direction reduces considerably, and the adhesion properties between the surface of the cathode layer of the capacitor element and the surface of the cathode terminal are improved. Unlike the conventional method, any air bubbles are not produced and, hence, the electric characteristics (tan xcex4/1 KHz or ESR/100 KHz) are improved. Moreover, the use of an adhesive that cures within a short time enables successive curing within the manufacturing apparatus, compared with the conventional batch processing by the use of a hot-air circulating dryer, thus enhancing the productivity.
Preferably, the pressure applied to the plurality of capacitor elements ranges from 2 kg/cm2 to 9.5 kg/cm2 in terms of leakage current.
Advantageously, an epoxy-based adhesive is used for the conductive adhesive, and the conductive adhesive has a thickness smaller than 100 xcexcm after the application of the pressure. The use of the epoxy-based adhesive enables curing within a very short time and enhances the productivity, and the reduced thickness of the conductive adhesive reduces the resistance of the conductive adhesive.
On the other hand, the apparatus according to the present invention includes a lower mold having an upper surface, on which a length of metallic lead frame having a plurality of anode terminals and a plurality of cathode terminals is to be placed, an upper mold mounted on the lower mold and having a lower surface spaced from the upper surface of the lower mold, and a heater mounted in the lower mold. The plurality of anode terminals are joined to respective anode leads extending outwardly from a plurality of capacitor elements, and the plurality of cathode terminals are coated with a thermosetting conductive adhesive, on which cathode layers of the plurality of capacitor elements are placed. When a pressure is applied to the plurality of capacitor elements by moving the upper mold toward the lower mold, a portion of the conductive adhesive is squeezed out of one surface of each of the plurality of capacitor elements to a neighboring side surface thereof, thus causing the conductive adhesive to have substantially a uniform thickness. Thereafter, the plurality of cathode terminals and the associated cathode layers of the plurality of capacitor elements are joined together by heat-curing the conductive adhesive.
The apparatus of the above-described construction facilitates the manufacture of the tantalum solid electrolytic capacitors and improves the quality thereof.
The upper mold moves up and down relative to the lower mold. Alternatively, the upper mold is hingedly connected to the lower mold so as to pivot about a fulcrum. Such movement of the upper mold simplifies the construction of the apparatus and enhances the reliability of the apparatus.
Advantageously, a water-repellent and heat-resistant sheet is attached to the upper surface of the lower mold. This sheet acts to prevent the cathode terminals from being bonded to the lower mold, even if the conductive adhesive is expelled from the cathode terminals, thus avoiding a reduction in productivity.
Again advantageously, a heat-resistant elastic material is attached to the lower surface of the upper mold. The elastic material allows the capacitor elements to be appropriately pressed without directly applying the pressure thereto.
It is preferred that a heater be mounted in the upper mold. The heater in the upper mold acts to prevent heat from radiating from the upper mold and to reduce the time required to reach the temperature at which the adhesive is cured.
It is also preferred that a thermal insulating panel be disposed adjacent to the lower mold so as to confront a side surface thereof. The thermal insulating panel acts to prevent heat radiation from a heat source and to stabilize the temperature of the lower mold.
Advantageously, a load regulating mechanism is provided for regulating a load to press the upper mold against the lower mold. The load regulating mechanism acts to optimize the load or pressure applied to the capacitor elements without damaging them.